The present invention is generally dealing with communication systems.
The present invention is in particular dealing with communication systems having a core network comprising a core network subsystem, such as in particular the IMS (Internet Protocol Multimedia Subsystem), accessed by an access network.
The example of IMS (as specified by 3GPP (3rd Generation Partnership Project)), will be considered more particularly in the following, as an example to which the present invention applies; however the present invention is not limited to such an example, and other examples are of course possible, such as for example MMD (Multi Media Domain, as specified by 3GPP2 (3rd Generation Partnership Project2)), IMS-NGN, . . . etc.
IMS is defined in particular in Technical Specification 3GPP TS 23.228 published by the 3GPP. As defined in this Technical Specification a system for the support of IP multimedia applications includes terminals, an IP-Connectivity Access Network (IP-CAN), and an IMS subsystem (IMS). An example of IP-CAN corresponds to the GPRS (General Packet Radio Service) Core Network with a Radio Access Network (or RAN) corresponding to GERAN (GSM EDGE Radio Access Network, where GSM stands for “Global System for Mobile communications” and EDGE stands for “Enhanced Data rates for GSM Evolution”) and/or UTRAN (Universal Terrestrial Radio Access Network).
The example of GPRS will be considered more particularly in the following, as an example to which the present invention applies; however the present invention is not limited to this example, and other examples are of course possible, such as for example WLAN (Wireless Local Area Network), broadband access x-Digital Subscriber Line, . . . etc.
The general architecture of a system comprising a IMS subsystem, accessed by an access network IP-CAN, is recalled in FIG. 1, where there is illustrated:                a terminal, or UE (User Equipment),        an access network IP-CAN including, among other elements not specifically illustrated, elements having a function of gateway with the IMS subsystem, such as in particular, in the example of GPRS, GGSN (Gateway GPRS Support Node), or, in the example of WLAN interworking, PDG (Packet Data Gateway),        an IMS subsystem including, among other entities not specifically illustrated, entities such as in particular P-CSCF (Proxy-Call Session Control Function), I-CSCF (Interrogating-Call Session Control Function), S-CSCF (Serving-Call Session Control Function), HSS (Home Subscriber Server), AS (Application Server).        
It is recalled that IMS uses a session control protocol based on the Session Initiation Protocol (SIP) as defined in particular in 3GPP TS 24.229. IMS entities such as P-CSCF, I-CSCF, S-CSCF correspond to SIP servers: P-CSCF corresponds to an entry point within the IMS, I-CSCF corresponds to an entry point within an operator's network, S-CSCF corresponds to an entity mainly in charge of session control and registration. HSS corresponds to a data base for subscriber and service-related data of the IMS, and ASs can be used to provide value-added services to a subscriber. Different IMS procedures, and the roles of the different IMS entities in these different IMS procedures, are defined in particular in 3GPP TS 23.228 and 3GPP TS 24.229.
It is also recalled that in the case of mobile access, the access network generally comprises different networks or PLMNs (Public Land Mobile Networks) between which the user may roam; further, the access network may support different Radio Access Technologies (RATs), and in the same way the terminals may be multi-RAT terminals, supporting different RATs. Examples of RATs include second generation technologies or 2G (based on TDMA, or Time Division Multiple Access), third generation technologies or 3G (based on W-CDMA, or Wideband-Code Division Multiple Access), UMA (Unlicensed Mobile Access), Wi-Fi . . . etc. In such a context, information enabling to indicate how an user accesses the IMS subsystem is generally necessary, for control of service delivery to an user, in the core network in general, and in the IMS subsystem in particular. In a general way, such information can be called user access-dependent information.
Such user access-dependent information may in particular include:                location information, enabling to describe the current location of the user within the network, generally with one or more parameters or identities such as (in the case of mobile access): the identity of a serving cell, the identity of a group of cells including a serving cell (such as LA (Location Area) or RA (Routing Area)), the identity of a serving network element, the identity of the serving PLMN where the user is actually camping (i.e. whether the user is roaming or located in his/her Home network), . . . etc.,        access technology information, enabling to describe the access technology currently used by the user, for example the Radio Access Technology (RAT) actually used by the user, such as for example GERAN, UTRAN, UMA, Wi-Fi, . . . etc.        
Such user access-dependent information may in particular be very useful for the following tasks:                charging: e.g. lower charge when user is under Wi-Fi coverage, charging depending on the user being roaming or in nomadic situation . . . etc.,        service authorization: e.g. to check whether a roaming UE is allowed to use an IMS service or to use IMS at all, i.e. to check whether the user is allowed to benefit from IMS service in the network where he/she is currently camping on,        management of emergency services,        . . . etc.        